Cook shallot in remaining tablespoon butter in a 10-inch heavy skillet over moderate heat, stirring, until softened, 2 to 3 minutes. Add mushrooms and cook, stirring, until lightly browned, about 4 minutes. Stir into velouté sauce along with chicken and sherry.

Cut tips from asparagus and set aside. Cut enough stalks into ¼ inch pieces to measure 1 cup, reserving remainder for another use. Cook tips in a saucepan of boiling salted water until crisp-tender, 2 to 4 minutes. Transfer to a bowl of ice and cold water with a slotted spoon. Scoop out tips and drain on paper towels, then reserve for topping. Cook and drain chopped asparagus in same manner and stir into velouté sauce with salt and pepper to taste.

Assemble and bake crêpes:

Preheat oven to 350 degrees.

Spread 1/4 cup filling across center of 1 crêpe and roll into a cylinder. Transfer, seam side down, to buttered baking dish, then assemble more crêpes in same manner, fitting them snugly. Bake in middle of oven 15 minutes.